A blue-water navy is basically a group of really big ships that go far away from land and stay out on the open water for a long time. It's like a very, very big version of the boats you might see sailing around in a lake or river.
These navy ships are really important because they can protect countries from other countries who might want to hurt them. They can also help keep trade routes safe so that countries can trade with each other and people can get the things they need.
Blue-water navies are named that way because they are designed to operate in blue water, which is the deeper and more open parts of the ocean that you can't see the bottom of. They're designed to be able to move around easily and quickly, and to protect themselves and the people on board from bad weather and attacks.
Being part of a blue-water navy is a really important and big job. People who work on these ships train really hard for a long time to make sure they know how to use all the equipment and do their jobs well. They also have to be ready to work together as a team to make sure everything on the ship runs smoothly and everyone stays safe.